Bottom Line:
In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction.As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm( (t) Bu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions.These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery).

ABSTRACTThe structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm( (t) Bu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nano-indentation on the major facets of the crystals shows that BF2dbm( (t) Bu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm( (t) Bu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.

fig3: Crystal packing in BF2dbm(OMe)2. (a) Face indices. (b) Crystal before (left) and after (right) mechanical shearing deformation. (c) Showing molecular arrangement with respect to the indentation direction (grey arrow). (d), (i) one-dimensional tape formed by C—H⋯(BF2O2). (ii) Partial representation of a two-dimensional sheet (molecules viewed from side). (e) Representation of the slip planes in crystal packing and the orientation of the indentation direction with respect to slip planes; the (010) plane (on which indentation has been done) is shown in red.

Mentions:
BF2dbm(OMe)2 is known to crystallize in the centrosymmetric monoclinic space group C2/c, with half a molecule in the asymmetric unit (Fig. S1(ii), for clarity the full molecule is shown), which is redetermined here (Fig. 3 ▸) (Yoshii et al., 2013 ▸). The bifurcated C—H⋯F (2.67 Å, 157.56°) and C—H⋯B (3.019 Å, 162.24°) intermolecular interactions between the phenyl and BF2O2 groups form linear tapes, which are further linked by additional C—H⋯F interactions (2.57 Å, 118.26°) to form two-dimensional sheets. In the crystal packing slip planes are formed by —OCH3 functional groups. In this case the crystals undergo plastic shearing deformation (Ghosh & Reddy, 2012 ▸; Reddy et al., 2010 ▸; Krishna et al., 2015 ▸; Bag et al., 2012 ▸) and do not show plastic bending on any of the faces.

fig3: Crystal packing in BF2dbm(OMe)2. (a) Face indices. (b) Crystal before (left) and after (right) mechanical shearing deformation. (c) Showing molecular arrangement with respect to the indentation direction (grey arrow). (d), (i) one-dimensional tape formed by C—H⋯(BF2O2). (ii) Partial representation of a two-dimensional sheet (molecules viewed from side). (e) Representation of the slip planes in crystal packing and the orientation of the indentation direction with respect to slip planes; the (010) plane (on which indentation has been done) is shown in red.

Mentions:
BF2dbm(OMe)2 is known to crystallize in the centrosymmetric monoclinic space group C2/c, with half a molecule in the asymmetric unit (Fig. S1(ii), for clarity the full molecule is shown), which is redetermined here (Fig. 3 ▸) (Yoshii et al., 2013 ▸). The bifurcated C—H⋯F (2.67 Å, 157.56°) and C—H⋯B (3.019 Å, 162.24°) intermolecular interactions between the phenyl and BF2O2 groups form linear tapes, which are further linked by additional C—H⋯F interactions (2.57 Å, 118.26°) to form two-dimensional sheets. In the crystal packing slip planes are formed by —OCH3 functional groups. In this case the crystals undergo plastic shearing deformation (Ghosh & Reddy, 2012 ▸; Reddy et al., 2010 ▸; Krishna et al., 2015 ▸; Bag et al., 2012 ▸) and do not show plastic bending on any of the faces.

Bottom Line:
In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction.As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm( (t) Bu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions.These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery).

ABSTRACTThe structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm( (t) Bu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nano-indentation on the major facets of the crystals shows that BF2dbm( (t) Bu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm( (t) Bu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.